1. Field of the Invention
The present invention relates to a camera that has a shutter movement state detecting device.
2. Description of the Related Art
With conventional cameras, light rays from a subject being photographed are controlled by the lens diaphragm, the length of exposure of the film being controlled by a mechanical shutter. The shutter used in single lens reflex cameras is basically composed of two curtains, i.e. a front curtain and a rear curtain, with the front curtain covering the film surface prior to exposure. When the shutter button is depressed, the front curtain first withdraws from the film surface, thereby starting the exposure of the film surface of light. After a set time has elapsed, the rear curtain moves to cover the film surface. The movement of the curtains is driven by the mechanical force of springs, and the start of movement is accomplished by releasing the stop of each curtain.
Heretofore, these actions were regulated through purely mechanical means, but in recent years electrical means have come to be widely used. The actual movement of the front and rear curtains is still accomplished in the same manner through the force of a spring, but the commencement of movement is regulated through a current to electromagnets that connect and hold the curtains.
With high speed shutter settings such as 1/8,000 of a second, a high strobe synchronization speed is necessary in order to make photography possible under bright conditions. In order to do so, it becomes necessary to rapidly boost the curtain movement speed by increasing the driving force of the described spring, and it also becomes necessary to narrow the width of the slit formed by the front curtain and the rear curtain.
However, even if the timing of electricity supplied to the electromagnet and the control of the exposure duration is accurate, the actual exposure duration is, as explained heretofore, dependent on the mechanical movement state of the curtains. This phenomenon is only further heightened by increasing the movement speed. The spring force had to be raised and the moving speed of each curtain (referred to hereafter as "curtain speed") had to be accelerated. Moreover, the width of the slits forming the front and rear curtains had to be narrowed.
Therefore, at high speeds above 1/8000 of a second, even if the electric current timing of the electromagnet is accurately regulated, there is a possibility that the desired shutter speed will not be obtained because of the separation speed of the mechanism that connects the electromagnet, and because there is some dispersion in the mechanical moving system that contains the spring. Furthermore, because this type of development is first discovered when the film is developed, a camera is offered that is constructed so as to have a shutter blade movement state detector built onto the shutter, and to be able to check the shutter time, a shutter timing device that can check the shutter speed even while photographing.
However, because the shutter blade movement state detector uses infrared rays (with a wavelength in the neighborhood of 920 nm), and because the detector is arranged in close proximity to the shutter aperture, there has been the problem of the infrared completely sensitizing the film that is in the position to be exposed.
In order to overcome such problems, Japanese unexamined patent application Hei 5-2231 reveals a method in which a detector is displaced inside the shutter that measures the actual shutter time and that issues a warning depending on the results of this measurement. In this instance, a photoelectric converter comprised of a light-emitting diode (LED) and a photoresistor (PTR), which is an example of a non-contact method, is used as the detector.
On the other hand, voltage check circuitry for the camera battery displaced within the camera is well known under the conventional art. When the battery voltage drops below a specified value, operation of the camera is prevented in order to prevent malfunctions, the primary purpose of the check circuitry being to provide a warning display that indicates the occurrence of the problem.
However, when the photoelectric converter described above is used as the shutter movement state detection system, its operation discharges the battery (power source).
In order to detect the short time interval corresponding to the faster shutter speeds, and in order to also narrow the slit formed by the front curtain and the rear curtain, it is necessary to increase the amount of light generated by the LED and to boost the operating speed of the PTR circuit, making increased power consumption by the detection circuit inevitable.
Accordingly, the operation of the shutter movement state detection system prevents the exposure operation comprising the main function of the camera, with the problem that the power source voltage drops during the exposure action, causing malfunctions to occur in the shutter movement state detection system.